The invention relates to a method of manufacturing a semiconductor device whereby a surface zone adjoining a surface is formed in a silicon semiconductor body in that carbon and dopant atoms are provided locally, the carbon atoms being provided through implantation.
Such a method is suitable for manufacturing a surface zone whose material after implantation with the carbon atoms has a greater bandgap than the silicon of the semiconductor body. The surface zone may be used, for example, as an emitter region in a heterojunction bipolar transistor (HBT) or as a barrier in a "tunnelling hot electron transistor". The dopant atoms serve to determine the conductivity type of the surface zone. The material of the surface zone is given a certain conductivity type: p type or n type, in that dopant atoms are provided by diffusion or implantation in usual manner before or after implantation of the carbon. If the material of the semiconductor body now has an opposite conductivity type, a hetero pn junction is created between the surface zone and the semiconductor body. Such a junction may function, for example, as an emitter-base junction of a HBT. Transistors provided with such a heterojunction are fast and very efficient.
U.S. Pat. No. 4,559,696 discloses a method of the kind mentioned in the opening paragraph whereby the surface zone is formed through implantation with arsenic and carbon ions at an energy of approximately 100 keV in a dose of approximately 4.multidot.10.sup.16 /cm.sup.2. A surface zone of n-type material having a greater bandgap than silicon is then created. The surface zone acts as the emitter of a HBT.
When a transistor with a heterojunction is made by the known method, heat treatments are found to be necessary in practice after the carbon implantation in order to restore damage in the base-emitter junction or in the base region, or to activate or diffuse the dopant atoms. After such a heat treatment, however, a number of recombination centers remains in the surface zone, which adversely affects the efficiency and the switching speed of the heterojunction transistor. It is further found in practice with heat treatments above approximately 800.degree. C. that the provided dopant atoms diffuse from the surface zone, so that a zone implanted with carbon and a zone implanted with the dopant atoms do not coincide. Thus it may happen as a result of the diffusion of dopant atoms from the surface zone, for example, that a junction between the surface zone of greater bandgap and the semiconductor body of smaller bandgap does not coincide with a junction between n-type and p-type material. Instead of a heterojunction, a normal pn junction would then be created in the case of an emitter-base junction of a HBT. It is also found to be difficult in practice to implant a comparatively shallow surface zone with carbon atoms because carbon atoms on account of their small size already penetrate deeply into a material during implantation at low implantation energies; thus a carbon atom penetrates a silicon semiconductor body by as much as 0.3 .mu.m deep at an implantation energy of 100 keV.